1. Field of the Invention
The present invention relates generally to non-volatile magnetic memory and particularly to multi-state magnetic memory having stacked magnetic tunnel junction (MTJ) with at least one MTJ having a storage layer with a magnetization direction being substantially perpendicular to the plane of the wafer (or substrate).
2. Description of the Prior Art
It has become clear that non-volatile memories, such as non-volatile random access memories (NVRAMs) are finding increasingly new applications, and that their notoriety would not only continue but that it would explode due to the increasing demands in mobile devices and other applications. It is also believed that current volatile DRAM will be replaced by the current-switching version or spin-torque transfer magnetic random access memory (STTMRAM), in the coming future.
It is really important that in order for STTMRAM to gain broad acceptance, the cost of the memory and hence the cell size to be smaller and scalable to smaller feature size such as below 65 nm.
One of the ways to achieve higher density is by storing more than one bit of digital information in a memory cell of magnetic memory designs. Such memory cells are commonly referred to as multi-state (or multi-level) cells (MLCs). Current MLC designs include magnetic memory with a storage layer with a magnetization direction that is in-plane or parallel to the plane of the substrate or wafer. Such MLCs suffer from low density and therefore higher costs.
Stated differently, current STTMRAM designs have storage layers with a magnetization direction that is parallel to the plane of the substrate. One of the key challenges for such type of designs using in-plane magnetization is that they cannot be scaled to lower feature sizes such as below 65 nm. This is primarily because as the feature size reduces, the reduced memory bit size (for example: for 90 nm, the memory bit size is 90 nm×180 nm, but for 45 nm this would be 45 nm×90 nm) becomes thermally unstable due to the reduction in the volume. The Perpendicular STTMRAM enables lower switching current (therefore lower program current), higher thermal stability and higher density.
Moreover, perpendicular STTMRAM does not require in-plane elongated memory bit like in the case of in-plane STTMRAM, mentioned above. In-plane circular shape or square shape is typically preferred for perpendicular STTMRAM over an elongated shape because in the latter, features pose large manufacturing challenges especially at lower lithography geometry, such as below 90 nm. On the other hand, circular shape memory bit shapes allow for lower lithography and higher capacity memory, making the perpendicular STTMRAM a preferred choice for future applications. It should be pointed out that the perpendicular STTMRAM can utilize high anisotropy magnetic alloys as compared to the STTMRAM having in-plane magnetization, leading to higher thermal stability and thereby enabling scalability down to below 30 nm.
What is needed is a multi-state current-switching version or spin-torque magnetic random access memory (STTMRAM) having stacked magnetic tunnel junctions (MTJs).